池沸腾临界热流密度及临界波长的实验研究

分别在光滑及波形结构的铜表面上对水和乙醇进行饱和池沸腾实验,观测了临界热流密度(CHF)下临界波长的变化趋势,并分析了表面结构对沸腾传热系数及CHF的影响。实验验证了光滑表面上,临界波长随工质的不同而变化,继而影响CHF,其实验值与经典的临界波长及临界热流密度理论一致。而粗糙表面上的乙醇沸腾实验进一步发现,波形结构可以减小临界波长,从而有效提高CHF,其影响规律与相关文献的理论模型较为符合。     

泡沫金属圆管内沸腾传热的数值模拟

为了解泡沫金属沸腾传热原理,建立了泡沫金属圆管三维物理模型,采用BrinkmanForchheimer扩展达西动量方程和C语言编写气液两相质量传递和能量传递的自定义函数,对泡沫金属圆管中沸腾传热现象进行数值模拟,分析了质量流率、干度对流型、压降和沸腾传热系数的影响。模拟结果表明,在一定质量流率下,单位长度压降随着干度的增大成非线性增长趋势;低质量流率时,随着干度的增大,管内流型由分层流过渡到波状流进而过渡到稳定的波状流,传热系数变小;高质量流率时,随着干度的增大,管内流型由弹状流过渡到环状流,传热系数变大。     

电化学方法制备多孔微纳结构表面的池沸腾传热性能研究

电化学表面处理一直以来都是池沸腾传热强化领域的一种重要的表面改性方法。然而,在光滑表面通过不断沉积生长枝晶结构的同时,结构底层会形成不必要的堆积层,从而增加了底层的热阻,不利于表面沸腾传热系数（HTC）的提升。因此,本文通过结合电化学表面处理手段中的电化学腐蚀与电化学沉积两方面的工艺,制备了“电腐蚀+沉积”表面,来探究经预先腐蚀处理后的表面与直接沉积表面沉积形貌及沸腾传热性能的差异。SEM图像表明,相较于在未处理铜板上直接进行沉积的表面,“电腐蚀+沉积”表面具有更长的枝晶结构、更深的孔结构及较少的底层堆积,结构的整体粗糙度更大。稳态池沸腾实验结果表明,在未处理铜板上直接进行电沉积的表面,其临界热流密度值较未处理铜板提升141%,而“电腐蚀+沉积”表面的临界热流密度值相较于未处理铜板提升了193%,并且其整体的沸腾曲线相较于直接沉积表面左移。对表面沸腾过程中汽泡行为的观测及表面铺展速率测试结果表明,“电腐蚀+沉积”表面具有更强的回液能力,能更好地促进表面气泡的脱离过程。     

均匀及梯度泡沫金属复合PCM熔化特性模拟研究

基于六面通圆孔的均匀泡沫金属结构,构建了泡沫金属复合相变材料(PCM)三维模型,采用高性能计算显卡(GPU)加速的多松弛时间格子玻尔兹曼方法模拟了均匀及梯度泡沫金属复合PCM的瞬态熔化过程。结果表明：随着均匀泡沫金属孔隙率的降低,复合PCM的传热速率提高,潜热储能的能力减弱;对于固定平均孔隙率的不均匀泡沫金属,孔隙率沿导热方向上递增的模型具有最佳的强化传热效果,其完全熔化时间比填充均匀骨架模型和孔隙率在导热方向上递减的骨架模型分别缩短了4.2%和25%,当孔隙率梯度变化方向与导热方向一致时,在高温壁面附近填充低孔隙率泡沫金属能显著强化传热;当两者方向垂直时,熔化速率取决于平均孔隙率,与梯度分布几乎无关。     

基于梯度孔隙率金属泡沫的复合相变单元储热性能数值模拟北大核心CSCD

向相变材料中添加金属泡沫可以解决相变材料低导热率引起的换热效果较差等问题,提高系统的整体蓄热效率。然而,复合相变材料的传热性能受金属泡沫孔隙率分布的影响较显著,为进一步提高相变储能单元的传热性能,本工作基于低孔隙率金属泡沫-相变材料(PCM)复合储能系统,建立了一种新的梯度孔隙率金属泡沫结构,通过数值模拟方法,对蓄热单元熔化过程中的熔化率、储能速率、储能总量进行分析,系统研究了孔隙率沿加热方向负梯度分布、正梯度分布对复合相变材料熔化速度和储热性能的影响。研究结果表明,负梯度孔隙率结构可以进一步提高储能系统的储热效率,其中,孔隙率梯度为0.12(案例S-6)时增强效果最显著。在熔化周期的不同阶段,负梯度孔隙率对复合材料的传热均有不同程度增强,对于S-6,在1000 s、2000 s、2600 s时,熔化率相较于均匀孔隙率结构分别增加了0.67%、2.31%、9.90%;随着孔隙率梯度的增加,相变材料的热性能提高越显著,与均匀孔隙结构相比,改进的负梯度孔隙率结构其完全熔化时间最高可缩短7.32%,储热速率可提高8.02%。对于正梯度孔隙率结构,其对熔化速度没有显著影响,但是储热总量可提高0.49%。     

强润湿性液体池沸腾传热的实验研究和机理分析

对强润湿性液体的池沸腾传热实验而言,本文提出了行之有效的实验程序,并严格按照实验程序进行了R113池沸腾传热的实验研究,具体研究了表现老化和液体过冷度对池沸腾传热曲线及起沸点的影响,实验中观察到了三个反常现象,最后,从强润湿性液体的沸腾传热机理的角度对其给出了相应的解释。     

高低热导率相间表面强化饱和池沸腾的格子玻尔兹曼模拟

采用格子玻尔兹曼方法模拟高低热导率相间表面的饱和池沸腾过程,研究不同表面高低热导率区域热导率比值、低热导率区域宽度和深度对沸腾换热性能的影响。对比均匀热导率表面与高低热导率相间表面的沸腾曲线发现：高低热导率相间表面的沸腾过程可被分为5个阶段,并且其临界热流密度最高可达均匀表面的12倍;高低热导率相间可促使表面维持一定的温度差异,从而保持明显的气液流动;随着低热导率区域宽度增大,气液分离更加明显,低热导率区域宽度存在一个最优值,其与毛细长度的量级接近;随着低热导率区域的深度增大,表面过热度的差异更加明显。     

不凝结气体对多壁碳纳米管换热表面的池沸腾性能的影响

为揭示不凝结气体对多壁碳纳米管（Multi-walled Carbon Nanotube, MWCNT）纳米结构表面核态池沸腾过程的影响,使用气体沉积法（Chemical Vapor Deposition, CVD）在硅表面制作MWCNT纳米结构表面,并使用光滑硅表面进行对比实验研究。实验操作中,将驱气前后的工作液体应用于两种表面的池沸腾实验,换热表面过热度控制在0℃-35℃,工作液体过冷度分为40℃和50℃。实验结果表明,液体中含气量的变化对MWCNT纳米结构表面影响较小,而对光滑硅表面的影响较大;对比硅表面,MWCNT纳米结构表面能够有效提升沸腾传热效果,对于驱气后的工作液体提升效果更为明显。     

多孔金属和方柱复合结构池沸腾数值模拟研究

针对一种多孔金属和方柱复合结构,采用双粒子分布函数格子Boltzmann方法对其池沸腾传热进行数值模拟研究。分别调整了下部方柱的柱宽/槽宽(W/D)、柱高H和上部多孔层的厚度,分析了不同结构的沸腾换热曲线和传热机理。流场和温度场分别用密度分布函数和温度分布函数来描述,底部固体采用定压边界条件。研究发现,方柱-多孔层复合结构可以有效的强化沸腾换热。结果表明,低热流密度下,具有较小的W/D的结构换热表现更好,高热流密度下,较大的W/D换热表现更好;换热效果随结构柱高H的增大而增加;多孔层厚度的变化对整体结构的强化效果影响不大。     

石墨烯纳米制冷剂核态池沸腾换热的实验研究

为分析单层石墨烯纳米片对核态池沸腾换热的影响机理,对基液为R141b、分散相为单层石墨烯纳米片的纳米制冷剂的核态池沸腾换热特征进行了测定,采用Hot Disk热物性分析仪和铂金板法分别测定了石墨烯纳米制冷剂的热导率和表面张力,采用接触角测量仪和扫描电子显微镜（SEM）观测了沸腾后加热表面的润湿性和形貌特征。实验中,单层石墨烯纳米片的质量百分含量（ω）为0.02%~0.50%,实验压力为一个标准大气压,热流密度为20~200 kW/m2。实验结果表明：单层石墨烯纳米片的加入,使制冷剂R141b的核态池沸腾换热得到强化;当ω=0.2%时,换热系数提高比例出现峰值,为57.7%。伴随ω的增加,石墨烯纳米制冷剂的热导率增大、表面张力减小,沸腾表面润湿性增强且微腔数先增后减,综合作用的结果导致存在一个最佳的单层石墨烯纳米片浓度（即ω=0.2%）使换热系数最高。     

Effects of surface wettability on nucleate pool boiling heat transfer for surfactant solutions

Experiments for pool boiling of deionised water and acetone with different surfactant, 95% sodium dodecyl sulfate (SDS), Triton X-100 and octadecylamine, have been conducted under atmospheric pressure to investigate the effect of surface wettability. The boiling curves for different concentrations of surfactant solution on both smooth and roughened surfaces were obtained. The results show that the addition of surfactant can enhance the water boiling heat transfer, and the enhancement is more obvious for SDS solution; but has little influence on the acetone boiling curve. While the roughened surface enhanced the heat transfer for Triton X-100 solution, it also decreased the heat transfer coefficient for SDS solution. All these can be explained by including the changing of surface wettability, which has been neglected for a long time and should be an important parameter influencing boiling heat transfer. By incorporating such effects, the modified Mikic-Rohsenow pool boiling model, we proposed, can predict these experimental data well.     

Effect of nanoparticles on CHF enhancement in pool boiling of nano-fluids

To investigate the characteristics of CHF (Critical Heat Flux) enhancement using nano-fluids, pool boiling CHF experiments of two water-based nano-fluids with titania and alumina nanoparticles were performed using electrically heated metal wires under atmospheric pressure. The results showed that the water-based nano-fluids significantly enhanced CHF compared to that of pure water. SEM (Scanning Electron Microscopy) observation subsequent to the pool boiling experiments revealed that a lot of nanoparticles were deposited on heating surface during pool boiling of nano-fluids. In order to investigate the role of the nanoparticle surface coating on CHF enhancement of nano-fluids, pool boiling CHF of pure water was measured using a nanoparticle-coated heater prepared by pool boiling of nano-fluids on a bare heater. It was found that pool boiling of pure water on the naonoparticle-coated heater sufficiently achieved the CHF enhancement of nano-fluids. It is supposed that CHF enhancement in pool boiling of nano-fluids is mainly caused by the nanoparticle coating of the heating surface.     

形貌对通孔金属泡沫辐射性能的影响

本文借助PerkinElmer 光学测试系统实验研究了不同形貌通孔金属泡沫的漫反射率和漫透射率。实验结果表明,材质对金属泡沫的漫反射率有重要的影响。铜泡沫的漫反射率随着孔密度的增大而减小,而镍泡沫的漫反射率变化趋势则相反。无论是铜泡沫还是镍泡沫,吸收率皆随着孔密度的增大而增大,消光系数随着孔密度的增大而增大。对于烧结有铜板的铜泡沫,吸收率随着孔隙率的增大而增大。     

Effect of tube diameter on pressure drop characteristics of refrigerant–oil mixture flow boiling inside metal-foam filled tubes

The objective of this study is to analyze the effect of tube diameter on pressure drop characteristics of refrigerant–oil mixture flow boiling in metal-foam filled tubes. Experiments on metal-foam filled tubes with an inner diameter of 7.9 mm were performed, and the analysis of the diameter effect was done based on the experimental data of 7.9 mm tubes together with those of 13.8 mm and 26.0 mm from literature. The research results show that, the pressure drop increases with increasing PPI, and the impact of PPI becomes insignificant as the tube diameter decreases. When the diameter decreases from 13.8 mm to 7.9 mm, the pressure drop decreases due to the incomplete cells and randomly chopped ligaments nearby tube wall, and the maximum decrements are 22% and 35% for 5 PPI and 10 PPI metal-foam filled tubes, respectively. A new pressure drop correlation was developed, and it agrees well with the experimental data for different diameter tubes.     

Wettability of heated surfaces under pool boiling using surfactant solutions and nano-fluids

The wettability of the heated surface under pool boiling of surfactant solutions and nano-fluids has been investigated. Tri-sodium phosphate (TSP, Na₃PO₄) solutions (0.01, 0.05, 0.1, 0.3, 0.5, 0.8 wt.%) and Aluminum oxide (Al₂O₃) nano-fluids (NF) (0.5, 1, 2, 4 vol.%) were prepared for experiments. Stainless steel (SUS 304) strips (30 × 30 × 3 mm) were heated by an alcohol lamp and quenched in the prepared solutions. Before complete quenching, when the surface temperature was 150 ± 10 °C (nucleate boiling region), the strip was taken out and excessive liquid on the surface was removed. Contact angles of pure water and the solutions on the quenched surface and fresh surface were measured. Contact angles of pure water on the quenched surfaces (5°–25°) were much smaller than those on the fresh surface (65°–70°). The solutions (TSP, NF) on the quenched surface shows the smallest contact angle (5°–15°). Surfaces deposited TSP and nano-particle could affect surface energy of the strips and enhance hydrophilicity of the surfaces. Several implications of the experimental results on the pool boiling CHF model and CHF enhancement using TSP and NF were discussed.     

Photographic study of pool boiling CHF from a downward-facing convex surface

Heat transfer measurements and photographic studies are performed to capture the detailed evolution of the liquid–vapor interface near critical heat flux (CHF) for a 90-degree downward-facing convex surface. The test surface, with a width of 3.2 mm and a 102.6-mm radius, consists of a series of nine heaters that dissipate equal power. Instrumentation within each heater facilitates localized heat flux and temperature measurements along the convex surface, and transparent front and back windows enable optical access to a fairly two-dimensional liquid–vapor interface. Near CHF, vapor behavior along the convex surface is cyclical, repeatedly forming a stratified vapor layer at the bottom of the convex surface, which stretches as more vapor is generated, and then flows upwards along from the surface. Subsequently, heaters at the bottom of the convex surface, followed by the other heaters, are wetted with liquid before the nucleation/coalescence/stratification/release process is repeated. This study proves that despite the pronounced thickening of the vapor layer as it propagates upwards along the convex surface, CHF always commences on the bottom of the surface.     

Effect of surface particle interactions during pool boiling of nanofluids

The pool boiling characteristics of nanofluids is affected by the relative magnitudes of the average surface roughness and the average particle diameter. In the present work, an attempt has been made to study the interactions between the nanoparticles and the heater surface. The experimental methodology accounts for the transient nature of the boiling phenomena. The boiling curves of electro-stabilized Al₂O₃ water-based nanofluids at different concentrations on smooth and rough heaters and the burn-out heat flux have been obtained experimentally. Extensive surface profile characterization has been done using non-intrusive optical measurements and atomic force microscopy. A measure of the surface wettability has been obtained by determining the advancing contact angle. These results give an insight into the relative magnitudes of dominance of the prevalent mechanisms under different experimental conditions. Boiling on nanoparticle coated heaters has been investigated and presented as an effective solution to counter the disadvantageous transient boiling behavior of nanofluids.     

Nucleate pool boiling heat transfer over a bundle of vertical tubes

An experimental investigation has been carried out to determine the heat transfer coefficient during pool boiling of water over a bundle of vertical stainless steel heated tubes of 19.0 mm diameter and 850 mm height. The p/D of bundle was 1.66 and was placed inside a glass tube of 100 mm diameter and 900 mm length. The data were acquired for the heat flux range of 2–32 kWm^− 2.